This invention relates to an additive for wellbore fluids. More specifically, it pertains to an additive for modifying the properties of a filtercake formed at the boundary between wellbore and formation.
For the production of hydrocarbon wells, boreholes are drilled into subterranean formations. Following standard procedures, a fluid is circulated during drilling from the surface through the interior of the drill string and the annulus between drill string and formation. The drill fluid also referred to as drilling mud is used to lubricate the drill bit. It also balances the formation pressure so as to prevent formation fluids to enter the borehole in an uncontrolled manner.
The industry distinguishes between largely three classes of drilling fluids: oil-based, water-based and so-called synthetic muds. Whereas oil-based muds are recognized for their superior qualities for most of the drilling operations themselves, they become increasingly undesirable due to their impact on the environment and stricter environmental legislation. Water-based muds are expected to replace oil-based mud as the drilling fluid of choice in major geographical areas.
Other fluid systems used in the oilfield industry are completion and workover fluids. By definition a completion or workover fluid is a fluid that is placed against the producing formation while conducting such operations as well killing, cleaning out, drilling in, plugging back, controlling sand, or perforating. Basic fluid functions are to facilitate movement of treating fluids to a particular point downhole, to remove solids from the well, and to control formation pressures.
Required fluid properties vary depending on the operation, but the possibility of formation damage is always an important concern. In recent years many new fluid systems have appeared, most due to the recognition of the high risk of reducing the productivity, or completely plugging certain sections of the producing zone, through contact with a foreign fluid.
A wellbore fluid typically contains a number of additives. Those additives impart desired properties to the fluid, such as viscosity or density. One class of additives is used as fluid loss agents to prevent the drilling fluid from entering into porous formations.
The basic mechanism of fluid loss control is generally the formation of a filtercake at the interface of the porous or permeable formation layers. As part of the drilling fluid is forced into the formation by the higher pressure within the wellbore, larger particles and additives are left behind and accumulate at the face of the formation. The filtercake thus formed can be regarded as a membrane that protects the formation from further invasion of wellbore fluids. Fluid-loss control agents are selected in view of their quality to form a competent filtercake.
High molecular weight, oil soluble additives have been widely used as diverting agents and in water based drilling and completion fluids. The additives help build an effective filtercake at the formation face and minimize damage. Under flowback conditions, the reservoir hydrocarbons readily dissolve the additive effectively creating holes in the filtercake and aiding formation cleanup. Having a hydrocarbon-induced breakdown of the filtercake avoids chemical intervention (clean-up) and hence can result in a very cost-effective method of drilling wells for hydrocarbon production.
Many different oil soluble additives can be used to control fluid loss. High molecular weight thermoplastic hydrocarbon resins are commonly used. U.S. Pat. Nos. 3,891,566 and 3,827,498 describe for example a mixture of two oil soluble resins, one being friable and the other pliable as fluid loss control agents and as diverting agents. The resin must be used where oil exists and therefore it cannot be used in dry gas wells or in wells with temperature above its softening point. Its high molecular weight and impurities can make it only partially soluble in hydrocarbons causing the residues to damage or block the formation.
It is therefore an object of the present invention to provide a novel class of fluid loss additives that are able to form a competent filtercake while being easily removable from hydrocarbon bearing formations.
According to one aspect of the present invention, there is provided a low molecular weight, high melting point, crystalline, oil soluble additive for use in wellbore fluids that include drilling, completion, workover, fracturing, acidizing, cementing fluids and the like.
The new additive is a wellbore-fluid additive which is preferably a ground crystalline material of melting point over 80xc2x0 C., preferably over 100xc2x0 C. which is readily soluble in produced hydrocarbons such as crude oil and lighter condensates, and which exhibits a molecular weight of less than 1000, and preferably less than 650.
Its particle size can be adjusted to bridge efficiently across different pore size formations and control its solubility rate. A preferred particle size range is 10000 to 1 micron.
Preferred examples of the low molecular weight crystalline additives are 1S-endo-Borneol, camphor or iodine. Other examples include beta carotene with a melting point of 184 degrees Celsius and a molecular weight of 537, lycopene (175; 537), cholesterol (150; 387), lanosterol (139; 426), and agnosterol (165:424). More preferred examples are Woolwax alcohols distillation residues (Lanolin wax), a derivative of wool grease comprised of high molecular alcohols (triterpene, aliphatic etc) and fatty acids (normal, isoacids etc) with melting point of 90 degrees Celsius and a molecular weight of approximately 425.
The invention reduces formation damage, flow initiation pressure, and increases the cleanup efficiency dependent on the formation type. Compared to other additives, the low molecular weight crystalline additive can provide some important advantages. Firstly, the conventional high molecular weight resins, during the initial stages of dissolution in produced hydrocarbons, can be plasticized by the oil to a tacky material. This tends to coalesce to a sticky mass and is very difficult to remove by dissolution. Secondly, the pressure required to start the backflow of oil through such filtercake can be high and therefore the resins have limited use in wells with low drawdown pressures. Thirdly, the level of impurities in the hydrocarbon resin can limit its cleanup. By contrast, the low molecular weight crystalline solids of this invention dissolve smoothly and quickly to give low viscosity solutions.
In a variant of the invention, it is envisaged to reduce the environmental hazards and nuisance caused be some of the new crystalline solids compounds by an encapsulating treatment.
Another aspect of the invention relates to using the novel additive as part of a well operation. Such application comprises the steps of injecting a wellbore fluid with the additive, letting the additive form a filter cake, reversing the flow by changing the wellbore pressure and dissolving the additive in the filtercake through the flowback of hydrocarbon from the formation.
These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the following detailed description and drawings.